In a number of applications in the automotive industry, high-strength structural members with low mass are required. Various composite materials have been proposed in the past as structural members, such as exotic light-weight alloys. In most applications, however, mass reduction must not be at the expense of strength and must be balanced against the cost of the product to the consumer. Thus, there is a need for maintaining or, more preferably, for increasing the strength of structural members without significantly increasing materials and labor costs.
The reinforcement of motor vehicle structural members through the use of composite materials is also known. For example, the present inventor has disclosed a number of metal/plastic composite structures for use in reinforcing motor vehicles components. In U.S. Pat. No. 4,901,500, entitled "Lightweight Composite Beam," a reinforcing beam for a vehicle door is disclosed which comprises an open channel-shaped metal member having a longitudinal cavity which is filled with a thermoset or thermoplastic resin-based material. In U.S. Pat. No. 4,908,930 entitled, "Method of Making a Torsion Bar," a hollow torsion bar reinforced by a mixture of resin with filler is described. The tube is cut to length and charged with a resin-based material.
In U.S. Pat. No. 4,751,249, entitled, "Reinforcement Insert for a Structural Member with Method of Making and Using the Same," a precast reinforcement insert for structural members is provided which is formed of a plurality of pellets containing a thermoset resin with a blowing agent. The precast member is expanded and cured in place in the structural member, in U.S. Pat. No. 4,978,562, entitled, "Composite Tubular Door Beam Reinforced with a Syntactic Foam Core Localized at the Mid Span of the Tube," a composite door beam is described which has a resin-based core that occupies not more than one-third of the bore of a metal tube.
In U.S. Pat. No. 4,019,301, entitled, "Corrosion-Resistant Encasement For Structural Members," a piling or other structure is disclosed in which an I-beam is encased in a housing into which a resin is poured.
In co-pending U.S. patent application Ser. No. 145,798 filed May 19, 1994, entitled "Composite Laminate Beam for Automotive Body Construction," a hollow laminate beam characterized by high stiffness-to-mass ratio and having an outer portion which is separated from an inner tube by a thin layer of structural foam is described. In co-pending U.S. patent application Ser. No. 08/448,627, filed May 23, 1995, entitled "Composite Structural Reinforcement Member," a W-shaped carrier insert reinforcement which carries a foam body is described for use in reinforcing a hollow beam.
In addition, a number of metal laminates constructions are known in which flat metal plates are bonded together by an intervening layer of resin. It is also known to form a metal laminate sheet for use as a floor panel member which comprises a pair of flat metal sheets having an intervening layer of asphalt or elastic polymer.
Although filling the entirety of a section with plastic foam does significantly increase section stiffness (at least when high-density foams are utilized), they also increase mass and thus part weight, which, as stated, is an undesirable feature in automotive applications. Moreover, although increasing the metal gauge of a section or adding localized metal reinforcements will increase stiffness, as the metal thickness increases, it is more difficult to form the part due to limitations of metal forming machines. Importantly, in many applications increasing metal gauge will not work effectively because mass and stiffness are increased proportionately, with no resultant change in the dynamic stiffness frequency of the part.
Finally, filling a section entirely with foam may be prohibitively expensive, creates a large heat sink and requires elaborate sealing operations to close access holes in the stampings.
Accordingly, it would be desirable to provide a low-cost technique for reinforcing a hollow structural member without proportionately increasing the mass. The present invention provides sections which have increased strength with no moderate increases in mass and without the use of high volumes of expensive resins. In many applications, the present invention reduces vibrations which cause unwanted "shake" of a component which is primarily subjected to bending rather than torsion.